Systems comprising a conductor with measured current, a magnetic circuit in the form of a magnetic core with a gap located around the conductor with the current, and a magnetic field sensor located in the magnetic core gap belong to the most popular current measuring systems. These systems operate without a feedback control loop. If the measuring system is furnished with a secondary winding located on the magnetic core, which winding is connected with a magnetic field sensor and with an additional power supply system which forces the flow of current in the secondary winding, then this is a system operating with a feedback control loop.
The above mentioned systems provide a satisfactory measuring range for a vast array of applications. Their feature is the representation of the value and waveform of the measured current in the conductor in the form of a voltage or current signal. The output signal of these measuring systems is proportional to the measured current, both for the direct component and the alternating component. The direct component means current of a value that is constant in a given, finite time interval, whereas an alternating component means current of a value alternating periodically in a given time interval. The traditional system with feedback is shown in the state of the art in U.S. Pat. No. 4,682,101. A conductor with a measured current is placed in the magnetic core window. The magnetic core is placed coaxially with the conductor and it has an air gap in which a magnetic field sensor is placed. The signal from the field sensor is supplied to a signal conversion/amplification system and then to a secondary winding wound on the core and further to the measuring system connected to the secondary winding.
The disadvantage of the above described solution is that if there is a large disproportion between the direct component and the alternating component of the measured current, by way of example, in the order of 10,000 times, a very precise measuring system of a great measurement resolution is required. In such systems, the measured signal from the direct component which is several orders of magnitude lower can be at the level of noise of the conversion system, which in the worst case makes it impossible to measure the direct component. In cases where direct component measurement is significant while a precise measurement of the alternating component is not required, the above described disadvantage is of basic importance.